1. Field of the Invention
This invention is concerned with temperature control devices and more specifically with a thermostatic temperature control for heating baths.
2. Description of the Prior Art
Thermostatic control of temperature using various types of temperatures sensitive switches is known. These switches provide for on-off control of the heaters with the temperature of a temperature-controlled environment e.g. a bath, moves outside certain prescribed limits. The disadvantage of this type of control is that the temperature tends to vary excessively because the heater is turned on full for either a small or a large deviation, from the set point. Conversely, the heater will be turned completely off when the upper set point is reached even though it would be desirable to maintain a small input of heat to balance the inevitable losses.
Another disadvantage of existing systems is that the power of the heater must be carefully chosen according to the thermal capacity of the bath. Thus, if a given heater is used in a smaller bath or if a different medium is used in the bath, for example oil instead of water, the heating rate will be different and this may cause excessive variations of bath temperature.
The use of solid state electronic control circuits makes the proportional control of the heaters possible. For example, a thermistor whose resistance varies with temperature may be used to control the triggering of a thyristor in series with the load and the a.c. supply so that the thyristor is triggered at different times during the A.C. cycle. Such a circuit is described, for example, in Source book of Electronic Circuits, McGraw-Hill, pages 755 and 762. The disadvantages of this type of circuit, however, is that radio frequency interference may be caused. The strength of this R.F. interference increases in proportion to the power of the equipment. When powerful systems are used in areas where sensitive electronic apparatus is being used, this may cause serious trouble.
In addition to the need to provide a proportional response to temperature deviations, there is also a need to provide a so-called "reset" response, that is, a response which is delayed by a factor which corresponds to the thermal response time of the bath. For example, a thermistor may detect a change in the temperature of a temperature-controlled bath and may trigger a response before the bath as a whole has responded to a previous response. This can be avoided by delaying the control responses by an amount of time corresponding to the time constant of the bath. The reset response can be used to correct minor, but long-duration deviations from the set-point since it essentially functions in response to the time integral of the temperature deviation.
Up to now, it has been possible to obtain both proportional and reset control of temperature only by using a separate circuit for each of the two control responses, this complicates the control devices and, moreover, decreases their reliability because of the greater number of separate circuit elements involved.
I have now devised a way of controlling the temperature of a temperature-controlled environment which combines reset and proportional control and which enables both control responses to be obtained from one circuit.
According to the present invention the method of controlling the temperature of a temperature-controlled environment comprises:
1. sensing the temperature of the environment, PA1 2. generating a first signal in proportion to a deviation of the temperature from a reference temperature, PA1 3. varying the duration of the deviation of a second signal from a reference level in response to the first signal, PA1 4. deriving from the second signal a third signal having a value which varies with time, PA1 5. controlling the supply of heat to the environment in response to the first and third signals.
In a preferred embodiment of the invention, the second signal is a square wave signal whose on/off ratio is varied in proportion to the temperature of the environemt. This square wave signal can be filtered to produce a signal with sloping edges (the third signal) which is then combined with the first signal to control the heat flow to the environment.
The first signal is normally a d.c. signal whose value is proportional to the temperature. This combines with the sloping signal to produce triggering of a switch at varying times, dependant upon the value of the d.c. signal, the slope of the sloping signal and the voltage needed to trigger the switch which controls the heat supply. For example, if a large temperature deviation is detected, a large d.c. signal is produced and this, in combination with the sloping edge signal will trigger the supply switch for longer periods of time so that a greater corrective response is made. In this way, proportional control is obtained. Reset response is preferably obtained by integrating or averaging the d.c. signal and varying the on time of the square wave in response to this time-averaged value. The averaging is perferably performed by means of a filter which has a time constant greater than the time constant of the temperature-controlled environment. The time-constant of the environment is, of course, the time taken for the environment to respond to a corrective control response e.g. input of heat.
The invention is described with particular reference to the thermostatic control of a bath but it is also useful for controlling the temperature of other temperature-controlled environments such as ovens, refrigerators, and rooms. It may be used to control environments which are held at an elevated temperature for example, by controlling the supply of electrical current to electrical heaters, or controlling the supply of steam, hot air or gas by means of suitable valves which can be operated by servo mechanisms actuated by this control circuit. With environments held at a temperature below ambient, the supply of refrigerant or coolant may be regulated by suitable servo valves actuated by the control circuit.
The circuit permits substantial electrical currents to be switched without creating R. F. interference. This is particularly useful in laboratories and other places which contain apparatus which is sensitive to stray R. F. fields. In addition, the apparatus permits a greater degree of latitude in choosing heater (or coolant) power appropriate to the thermal cpacity of a given environment. This is made possible by the particularly efficient proportional and reset control embodied in the circuit.
An alarm system may be provided to provide a warning if the temperature of the bath deviates from the desired setting for more than a few minutes. A suitable circuit operates on the principle that when the bath is operating properly at the set point, the control heater operates about half the total time. The proportion of the time during which the control heater is on can be summed by an integrator and averaged over a period of about several minutes. If the amount of time during which the heater is on deviates intolerably from the ideal half, an output signal is produced and indicates that the deviation is higher than a preset limit or lower than a preset limit so that appropriate correction action can be taken.
The temperature control unit is particularly useful for controlling the temperature of viscometer baths, particularly the bath for an automatic viscometer of the type described in U.S. Pat. No. 3,798,960, the disclosure of which is incorporated herein by reference. The viscometer bath may be heated by two heaters -- a main heater to compensate for most of the thermal losses and a control heater to provide close control over the temperature. As this viscometer may use a computer for control of the various functions, it is particularly desirable to use the same computer for monitoring the operation of the present control circuit.